The invention relates to a method and device for controlling a display matrix screen adapted to display images having gray levels. It applies more particularly to the control of microdot fluorescent screens or liquid crystal screens. The images can be in black and white or in colour, the term gray level meaning in the latter case colour halftone.
To control the displaying of images on a matrix screen, the following method of sweep is generally used: the lines are successively addressed--i.e., taken from one appropriate potential Vlp to another appropriate potential Vla--once per image and for a time T (line time) which is identical for all the lines and is equal to the quotient of the duration of an image by the number of lines; simultaneously with the addressing of each line, the columns receive signals allowing the control of the respective states of the image elements, or pixels, of the line in question, as a function of the required image: a column is taken to an appropriate potential Vca if the corresponding pixel is to be illuminated, and to another appropriate potential Vce if on the other hand the corresponding pixel is to be extinguished. At the end of the time T, the addressing of the line in question ceases and the following line is addressed, the signals received by the columns depending on the respective required states of the pixels of this following line, and so on.
Techniques are also known allowing the production of images comprising gray levels:
A first technique consists in subjecting a column to a potential intermediate between Vca and Vce, so that the corresponding pixel has an intermediate brightness between that corresponding to the illuminated pixel and that corresponding to the extinguished pixel.
However, more particularly in the case of a microdot fluorescent screen, it is very difficult to control an intermediate voltage between Vca and Vce for a given brightness. because of the rigidity of the voltage/brightness characteristic of such a screen.
The second technique consists in taking a column to the potential Vca for only a fraction of the line time proportional to the quantity of light required for the corresponding pixel and in then returning the column to the potential Vce for the remainder of the line time (time modulation of the control potential of each column).
However, the relation between the time of application of Vca and brightness is not fully linear and, more particularly in the case of a microdot fluorescent screen, there is a strongly non-linear relation between the time of application and brightness, because of the time for establishing the voltage at the terminals of a pixel.
Moreover, in the case of one or other of the two aforementioned known techniques, the time for establishing the voltage at the terminals of a pixel also depends on the resistance of access to such pixel connected with its position in the screen. Consequently, the charge time of the pixel also depends on that position: for the same control potential two pixels, for example, situated at the two ends of the same column do not have the same brightness, the pixel closest to the column contact to which the control potential is applied having the strongest brightness.